This invention relates to heater controllers for oxygen sensors, and more particularly to a heater controller for an oxygen sensor associated with a vehicle engine and provided with a heater to maintain the oxygen sensors in a preferable activated state.
In engines for vehicles, oxygen (exhaust) sensors are provided in the exhaust system to control air-fuel ratio based on measured concentrations of oxygen in the exhaust gas for feedback. Some oxygen sensors are provided with heaters to maintain elements in a preferable activated state. The heaters are operated by controllers so as to be turned on at 100% of duty value and turned off at 0% of duty value.
Examples of heater controllers for oxygen sensors are disclosed in, e.g., JP Laid-Open Nos. H11-218044, H08-220059, and H05-202785.
According to JP Laid-Open H11-218044, a heater is energized when an ignition switch is turned on, and its duty is controlled according to both engine load and elapsed time after turning on the ignition switch so that the oxygen sensor reaches appropriate temperature quickly.
According to JP Laid-Open H08-220059, heater resistance is guarded at a higher first value in heating for maintaining the sensor in the preferable activated state, and is guarded at a second value that is lower than the first value after completion of the warming up of the air-fuel ratio sensor.
According to JP Laid-Open H05-202785, a heating sensor element is heated by setting a target element temperature to a high level if the engine temperature is relatively low, or by setting the target element temperature to a relatively low level if the cooling water temperature is relatively high.
In the conventional heater controller for the oxygen sensor according to, e.g. JP H11-218044, the heater for the oxygen sensor is controlled when the engine starts, that is the heater is energized when the ignition switch is turned on. The heater is controlled firstly according to the engine load, secondly by a map of engine speed and engine load, thirdly by the intake temperature and fourthly by elapsed time. The heater is controlled not only when the ignition switch is turned on to energize the heater, but even if the engine is started and the heater is energized, the above-mentioned first to fourth conditions are satisfied.
Moreover, the heater is energized at the instant when the ignition switch is turned on or the engine is started. As shown in FIG. 6 of JP Laid-Open H11-218044, the duty value for the heater is set as high as possible to expedite the activation of the oxygen sensor. However, condensation can form inside the exhaust system until the temperature of the exhaust system toward the upstream side of the oxygen sensor becomes about 50xc2x0 C. Exposing the oxygen sensor element that is heated by the heater to moisture can undesirably effect or damage the oxygen sensor element. Generally, if the temperature of the oxygen sensor element increases over 300xc2x0 C. before the temperature of the exhaust system toward the upstream side of the oxygen sensor reaches about 50xc2x0 C., then the condensation or moisture can damage the oxygen sensor element upon contact therewith. Accordingly, it is conventional to permit the heater to be energized instantly after the engine starts only if the engine water temperature is over about 20xc2x0 C.
Further, the heater is not conventionally energized when the engine water is at a low temperature to avoid damage to the oxygen sensor element. However, if the heater is not energized at all, the activation of the oxygen sensor is delayed. Feed back control for fuel is not executed until the oxygen sensor is activated, so that the fuel is injected under base control. Since the air-fuel ratio is generally set toward a rich mixture until the engine water temperature is increased after the engine starts, the efficiency of purification of the catalyst or catalytic converter is reduced and emits much undesirable exhaust gas that is not purified by the catalyst.
Still further, in the prior heater control, the heater is prevented from being energized so as to avoid damaging of the element. This delays the activation of the oxygen sensor. Output of emissions or exhaust gas is undesirably increased when the engine is started with a low temperature of the engine water (i.e. coolant). Depending on the construction of the engine exhaust system, when the exhaust system temperature is over a predetermined value (EXTL) (i.e. about 50xc2x0 C.), moisture inside the components of the exhaust system is evaporated, and the oxygen sensor is not damaged and allows the heater to be energized.
More particularly, as shown in FIG. 14, for CASE 1 showing the temperatures of the oxygen (exhaust) sensor element, when the temperature of the exhaust sensor element is higher than that of a predetermined value EXTH (about 300xc2x0 C.) (P1) before the temperature of the exhaust system (including exhaust pipe) increases to a predetermined value EXTL (about 50xc2x0 C.), the oxygen sensor element may be damaged. In other words, moisture on the sensor element can damage the sensor element when the temperature of the exhaust system is below the predetermined value EXTL (about 50xc2x0 C.) and when the temperature of the exhaust sensor is higher than the predetermined value EXTH (about 300xc2x0 C.). For CASE 2, the sensor element temperature is lower than the predetermined value EXTH (about 300xc2x0 C.) when the temperature of the exhaust system reaches the predetermined value EXTL (about 50xc2x0 C.) (P2), and is higher than the value EXTH (about 300xc2x0 C.) when the exhaust system temperature is higher than the value EXTL (about 50xc2x0 C.) (P3), thereby preventing moisture and the resulting damage to the sensor element.
FIG. 15 illustrates a conventional control of a heater according to the engine water (i.e. coolant) temperature. The heater is not energized at temperatures between the engine water temperature of xe2x88x9210xc2x0 C. at startup of the engine and an engine water temperature (0xc2x0 C.) at which the heater is turned on (P4). The engine water temperature is hardly increased at startup of the engine. During the time period (P5 to P6) when the exhaust system reaches the desired temperature (i.e., 50xc2x0 C. or above), the heater is not energized even if it could be. The oxygen sensor is not activated during this period, and the feed back of the air-fuel ratio is not executed, thereby increasing the output of the undesirable exhaust gases. Also, the lower the engine water temperature at startup, the longer the time for the exhaust system temperature to reach the predetermined value (EXTL; i.e. about 50xc2x0 C.), and the longer the time for the exhaust system temperature to reach a heater energy start temperature (i.e. 50xc2x0 C.). The heater is not energized in this case, which increases the period of time that the exhaust gas is emitted, and feed back control of the air-fuel ratio is delayed thereby decreasing exhaust purification efficiency.
To obviate or at least minimize the above inconveniences, the present invention provides a heater controller and control method for an oxygen sensor which detects concentrations of oxygen in exhaust gas from a vehicle engine and is controlled by a heater which has its duty (i.e., its electrical energization) controlled. The controller includes a start detect section, a restart determination section, and a heater delay control section. The start detect section detects whether the engine is started. The restart determination section determines whether certain parameters or conditions have been reached, that commonly indicate that the engine was recently restarted. The heater delay control section operates the heater to activate after a predetermined delay time, delayed from startup of the engine, that is set according to engine water temperature when the start detect section detects that the engine is started and when the restart determination section determines that the engine restart parameters have not been reached, also referred to as xe2x80x9cnot restarted.xe2x80x9d
According to the present invention, the heater is activated after the predetermined delay time that is set according to the engine water temperature after startup of the engine when it is determined that the engine is not restarted. That is, the delay time is set according to the engine water temperature at startup of the engine. Start of the heater control is delayed from the startup of the engine, which can also be set not according to the engine water temperature but according to a warm up state of the engine. The activation of the oxygen sensor is achieved at an early stage without damaging the structure thereof. The feed back control of the air-fuel ratio is achieved at an early stage to prevent increases in the exhaust gas output and thus improve the efficiency of exhaust gas purification.